The regional cerebral metabolic rate for glucose, rCMRglc, was measured with the 2-deoxy-D-glucose technique in 2 month and 24 month old male Fischer-344 rats, following administration of arecoline, a cholinergic agonist. The absence of age differences in most brain areas indicated that muscarinic post-synaptic receptor mechanisms are intact in the rat brain during aging. Dopaminergic function in the rat brain was examined by measuring rCMRglc in response to bromocriptine (a dopaminergic agonist) and sulpiride (a specific antagonist). The response to bromocriptine was reduced in senescent as compared to younger rats, suggesting a reduced central dopaminergic function, and an imbalance between cholinergic and dopaminergic systems. Regional cerebral blood flow (rCBF) was age invariant in awake Beagles between 1 and 12 years of age, and declined only in extreme senescence in relation to systemic disease, suggesting that cerebral functional activity is maintained during the life span of the healthy dog.